Alumina layer with enhanced texture

ABSTRACT

A refined method to produce textured α-Al 2 O 3  layers in a temperature range of from about 750 to about 1000° C. with a controlled texture and substantially enhanced wear resistance and toughness than the prior art is disclosed. The α-Al 2 O 3  layer is deposited on a bonding layer of (Ti,Al) (C,O,N) with increasing aluminium content towards the outer surface. Nucleation of α-Al 2 O 3  is obtained through a nucleation step being composed of short pulses and purges consisting of Ti/Al-containing pulses and oxidizing pulses. The α-Al 2 O 3  layer according to this invention has a thickness ranging from about 1 to about 20 μm and is composed of columnar grains. The length/width ratio of the alumina grains is from about 2 to about 12, preferably from about 5 to about 8. The layer is characterized by a strong (116) growth texture, measured using XRD, and by low intensity of (012), (110), (113) (024) and diffraction peaks.

BACKGROUND OF THE INVENTION

The present invention relates to a coated cutting tool insert designedto be used in metal machining. The substrate is cemented carbide,cermet, ceramics or cBN on which a hard and wear resistant coating isdeposited. The coating exhibits an excellent adhesion to the substratecovering all functional parts thereof. The coating is composed of one ormore refractory layers of which at least one layer is a stronglytextured alpha-alumina (α-Al₂O₃) deposited in the temperature range offrom about 750 to about 1000° C.

A crucial step in the deposition of different Al₂O₃ polymorphs is thenucleation step. κ-Al₂O₃ can be grown in a controlled way on {111}surfaces of TiN, Ti(C,N) or TiC having the fcc structure. TEM hasconfirmed the growth mode which is that of the close-packed (001) planesof κ-Al₂O₃ on the close-packed {111} planes of the cubic phase with thefollowing epitaxial orientation relationships: (001)_(κ)//(111)_(TiX);[100]_(κ)//[112]_(TiX). An explanation and a model for the CVD growth ofmetastable κ-Al₂O₃ have proposed earlier (Y. Yoursdshahyan, C. Ruberto,M. Halvarsson, V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist,Theoretical Structure Determination of a Complex Material: κ-Al₂O₃ , J.Am. Ceram. Soc. 82 (6) (1999) 1365-1380.

When properly nucleated κ-Al₂O₃ layers can be grown to a considerablethickness (>10 μm). The growth of even thicker layers of κ-Al₂O₃ can beensured through re-nucleation on thin layers of, for example TiN,inserted in the growing κ-Al₂O₃ layer. When nucleation is ensured theκ→α transformation can be avoided during deposition by using arelatively low deposition temperature (<1000° C.). During metal cuttingthe κ→α phase transformation has confirmed to occur resulting in flakingof the coating. In addition to this there are several other reasons whyα-Al₂O₃ should be preferred in many metal cutting applications. As shownearlier α-Al₂O₃ exhibits better wear properties in cast iron (U.S. Pat.No. 5,137,774).

However, the stable α-Al₂O₃ phase has been found to be more difficult tobe nucleated and grown at reasonable CVD temperatures than themetastable κ-Al₂O₃. It has been experimentally confirmed that α-Al₂O₃can be nucleated, for example, on Ti₂O₃ surfaces, bonding layers of(Ti,Al) (C,O) or by controlling the oxidation potential using CO/CO₂mixtures as shown in U.S. Pat. No. 5,654,035. The bottom line in allthese approaches is that nucleation must not take place on the111-surfaces of TiC, TiN, Ti(C,N) or Ti(C,O,N), otherwise κ-Al₂O₃ isobtained.

It should also be noted that in the prior-art methods higher depositiontemperatures (about 1000° C.) are usually used to deposit α-Al₂O₃. Whenthe nucleation control is not complete, as is the case in many prior-artproducts, the produced α-Al₂O₃ layers have, at least partly, been formedas a result of the κ-Al₂O₃→α-Al₂O₃ phase transformation. This isespecially the case when thick Al₂O₃ layers are considered. These kindsof α-Al₂O₃ layers are composed of larger grains with transformationcracks. These layers exhibit much lower mechanical strength andductility than the α-Al₂O₃ layers that are composed of nucleatedα-Al₂O₃. Consequently, there is a need to develop techniques to controlthe nucleation step of α-Al₂O₃.

The control of the α-Al₂O₃ polymorph in industrial scale was achieved inthe beginning of the 1990's with commercial products based on U.S. Pat.No. 5,137,774. Later modifications of this patent have been used todeposit α-Al₂O₃ with preferred layer textures. In U.S. Pat. No.5,654,035 an alumina layer textured in the (012) direction and in U.S.Pat. No. 5,980,988 in the (110) direction are disclosed. In U.S. Pat.No. 5,863,640 a preferred growth either along (012), or (104) or (110)is disclosed. U.S. Pat. No. 6,333,103 describes a modified method tocontrol the nucleation and growth of α-Al₂O₃ along the (10(10))direction. US20020155325A1 describes a method to obtain a strong (300)texture in α-Al₂O₃ using a texture modifying agent (ZrCl₄). Theprior-art processes discussed above use all high deposition temperaturesof about 1000° C.

US 2004/0028951A1 describes a new state-of-the-art technique to achievea pronounced (012) texture. The commercial success of this kind ofproduct demonstrates the importance to refine the CVD process of α-Al₂O₃towards fully controlled textures.

It is well established that the water gas shift reaction, in the absenceof H₂S or other dopants, is the critical rate-limiting step for Al₂O₃formation, and to a great extent, controls the minimum temperature atwhich Al₂O₃ can be deposited. Further it is well established that thewater-gas shift reaction is very sensitive for deposition pressure.

Extensive work has been done to deposit CVD Al₂O₃ at lower temperatures.Several Al₂O₃ layers using other than AlCl₃—CO₂—H₂ system have beeninvestigated, including AlCl₃—CO—CO₂, AlCl₃—C₂H₅OH, AlCl₃—N₂O—H₂,AlCl₃—NH₃—CO₂, AlCl₃—O₂—H₂O, AlCl₃—O₂—Ar, AlX₃—CO₂ (where X is Cl, Br,I), AlX₃—CO₂—H₂ (where X is Cl, Br, I), AlBr₃—NO—H₂—N₂ andAlBr₃—NO—H₂—N₂. It is emphasised that these studies have been carriedout without dopants (such as H₂S) and the effect of the depositionpressure has not been elucidated.

It is worth noticing that none of these latter systems have beencommercially successful. Consequently, to provide a CVD process fordepositing Al₂O₃ layers at temperatures below those currently used on acommercial scale is therefore highly desirable.

U.S. Pat. No. 6,572,991 describes a method to deposit γ-Al₂O₃ at lowdeposition temperatures. This work clearly shows that it is possible toobtain Al₂O₃ layers in the medium temperature range from theAlCl₃—CO₂—H₂ system. However, in this work it was not realised thatnucleation surface controls the phase composition of Al₂O₃ and thatdeposition of α-Al₂O₃ is thus possible at lower deposition temperatures.In the prior-art, it was considered impossible to deposit α-Al₂O₃ at lowtemperatures and it was believed that γ-Al₂O₃ and κ-Al₂O₃ were theunavoidable low temperature phases.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a new, improvedalumina layer where the α-Al₂O₃ phase consists of nucleated α-Al₂O₃ witha strong, fully controlled (116) growth texture. According to thepresent invention α-Al₂O₃ with the controlled (116) texture can beobtained within a wide temperature range from 750 to 1000° C., which canbe considered surprising.

In one aspect of the invention, there is a cutting tool insert of asubstrate at least partially coated with a coating with a totalthickness of from 10 to about 40μm one or more refractory layers ofwhich at least one layer is an alumina layer wherein said alumina layeris composed of columnar α-Al₂O₃ grains with texture coefficients

-   -   a) TC(116) greater than about 1.8    -   b) TC(012), TC(104), TC(110), TC(113), TC(024) all less than        about 1.5.

The texture coefficient TC(hkl) being defined as${{TC}\left( {{hk}\quad 1} \right)} = {\frac{I\left( {{hk}\quad 1} \right)}{I_{o}\left( {{hk}\quad 1} \right)}\left\{ {\frac{1}{n}{\sum\frac{I\left( {{hk}\quad 1} \right)}{I_{O}\left( {{hk}\quad 1} \right)}}} \right\}^{- 1}}$

where

-   -   I (hkl)=measured intensity of the (hkl) reflection    -   I_(o) (hkl)=standard intensity according to JCPDS card no        46-1212    -   n=number of reflections used in the calculation (hkl)        reflections used are: (012), (104), (110), (113), (024), (116).

In another aspect of the invention a method of coating a substrate withan Al₂O₃ layer wherein the α-Al₂O₃ layer is composed of columnar α-Al₂O₃grains with a texture coefficient TC (116) greater than about 1.8comprising depositing a (Ti,Al) (C,O,N) bonding layer on said substrateto provide a nucleation surface for said Al₂O₃, subjecting saidnucleation surface to a modification treatment of a pulse treatment witha mixture of TiCl₄, AlCl₃ and H₂, a purge with a neutral gas and anoxidizing pulse of a gas mixture including N₂ and CO₂, repeating themodification treatment and depositing α-Al₂O₃ having a texturecoefficient TC(116) greater than about 1.8 at a temperature of fromabout 750 to about 1000° C.

The alumina layer with strong texture outperforms the prior art withrandom or other less developed and incompletely controlled textures.Further, increased toughness can be obtained when deposition is carriedout at lower temperatures. Compared with prior-art products, the α-Al₂O₃layer according the present invention is essentially free fromtransformation stresses, consisting of columnar, defect free, α-Al₂O₃grains with low dislocation density and with improved cuttingproperties.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a cross-section SEM image (magnification 10000×) of atypical alumina layer according to the present invention deposited on aMTCVD-Ti(C,N) layer. The alumina layer is composed of columnar grains.It is dense with no detectable porosity.

FIG. 2 shows a cross-section SEM image of a typical layer according theprior-art (magnification 6000×) deposited on a MTCVD-Ti(C,N) layer. Thealumina layer is composed of large nearly equiaxed grains. Porosity isvisible in the alumina layer. Interfacial porosity between the aluminalayer and the Ti(C,N) layer is also visible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A method to deposit α-Al₂O₃ with a strong (116) texture in a temperaturerange of from about 750 to about 1000° C. is described. The invention isutilizing short pulses of precursors followed by purging steps with aninert gas such as Ar. After the purge another precursor is applied as ashort pulse. In addition to the texture control the method can be usedto produce finer grain sizes by increasing the number of nucleationsites. The texture-controlled α-Al₂O₃ layers deposited at mediumtemperature (about 800° C.) show enhanced toughness.

Al₂O₃ layers according to this invention outperform the prior-art andare especially suitable be used in toughness demanding stainlesssteel-cutting applications such as interrupted cutting, turning withcoolant and especially intermittent turning with coolant. The otherapplication area is in the cutting of cast iron where the edge strengthof this kind of alumina layer is superior to the prior art.

Ti(C,N) layer is used as an intermediate layer, which can be obtainedeither by conventional CVD or MTCVD, preferably by MTCVD. This inventionmakes it possible to deposit α-Al₂O₃ at same temperature as is used todeposit the intermediate MTCVD Ti(C,N) layer. Consequently, theheating-up period can be omitted after MTCVD.

To nucleate α-Al₂O₃ with the specified (116) texture several steps areneeded. First, on the Ti(C,N) layer a bonding layer characterised by thepresence of an Al concentration gradient is deposited. Nitrogen andCH₃CN are applied during deposition of this bonding layer. The aluminiumcontent on the surface of this layer being considerably, about 30%,higher than in the bonding layer according to U.S. Pat. No. 5,137,774(prior-art) and the bonding layer is obviously containing nitrogen. Thesurface of this bonding layer is subjected to an additionaltreatment(s).

Nucleation is started with a TiCl₄/AlCl₃/H₂ pulse with high TiCl₄content (greater than about 10%) and with a duration of about 5 minutes.After that an Ar purge (duration of about 5 minutes) is applied in orderto remove excess Cl⁻ from the surface. After this an oxidizing pulse isapplied using a CO₂/H₂/N₂/Ar (CO₂=about 0.15%, H₂=about 10%, N₂=fromabout 15 to about 17%, Ar=balance) gas mixture. The oxidizing step hasto be relatively short, from about 0.5 to about 5 minutes to secure(116) nucleation. The key to obtain the specified growth texture is thecontrol of the oxidation potential of the CO₂/H₂/N₂/Ar mixture byadjustment of the N₂:CO₂ ratio. This ratio should be from about 250 toabout 400, preferably from about 300 to about 350. The use of acontrolled oxygen potential in combination with the correct time andnumber of pulses enables the correct nucleation mode. Typical pulsetimes may range from about 0.5 to about 5 minutes depending on theduration of the pulse. The oxidizing pulse is again followed by an Arpurge. These steps can be repeated several times, preferably from about2 to about 5 times, in sequence to increase the amount of α-Al₂O₃nuclei. Too low or excessive oxidization must be avoided. A personskilled in the art can find the best and optimized combination betweenthe duration and the amount of the steps.

Detailed Description of the Nucleation Steps

-   -   1. Depositing a bonding layer from about 0.1 to about 1 μm thick        in a gas mixture of from about 2 to about 3% TiCl₄ and AlCl₃        increasing from about 0.5 to about 6%, from about 3 to about 7%        CO, from about 1 to about 3% CO₂, from about 0.2 to about 1.0%        CH₃CN from about 0.2 to about 1.0%, from about 2 to about 10% N₂        and balance H₂ at about 750 to about 1000° C., preferably at        about 800° C. and at a pressure of from about 50 to about 200        mbar.    -   2. Purging by Ar for about 5 min.    -   3. Treating the bonding layer in a gas mixture of from about 5        to about 15% TiCl₄ and from about 2 to about 4% AlCl₃ in        hydrogen for about 2 to about 60 min at about 750 to about 1000°        C., preferably at about 800° C. and at a pressure of about 50 to        about 200 mbar.    -   4. Purging by Ar for about 5 min.    -   5. Treating in a gas mixture of from about 0.1 to about 0.15%        CO₂ (preferably about 0.15%), from about 10 to about 30% N₂        (preferably from about 22.5 to about 30% when the CO₂ content is        about 15%), about 10% H₂, balance Ar at a pressure of from about        50 to about 200 mbar for about 0.5 to about 5 minutes at a        temperature of from about 750 to about 1000° C., depending on        the temperature for the subsequent deposition of the alumina        layer.    -   6. Purging by Ar for about 5 min.    -   7. Repeating steps 3-6 to obtain the optimum oxidization level.    -   8. Depositing an alumina layer at a temperature of from about        950 to about 1000° C. with desired thickness according to known        technique or depositing an alumina layer at about 750 to about        950° C. using higher deposition pressures (from about 200 to        about 500 mbar) together with higher amounts (from about 0.5 to        about 1.5%) of catalyzing precursors such as H₂S or SO_(x),        preferably H₂S.    -   The growth of the alumina layer onto the nucleation layer is        started by sequencing the reactant gases in the following order:        CO, AlCl₃, CO₂. The process temperatures of from about 750 to        about 1000° C. can be used since the texture is determined by        the nucleation surface.

The present invention also relates to a cutting tool insert of asubstrate at least partially coated with a coating with a totalthickness of from about 15 to about 40 μm, preferably from about 20 toabout 25 μm, of one or more refractory layers of which at least onelayer is an alpha alumina layer. The α-Al₂O₃ layer deposited accordingto this invention is dense and exhibits a very low defect density. It iscomposed of columnar grains with a strong (116) texture. The columnargrains have a length/width ratio of from about 2 to about 15, preferablyfrom about 5 to about 8.

The texture coefficients (TC) for the α-Al₂O₃ according to thisinvention layer is determined as follows:${{TC}\left( {{hk}\quad 1} \right)} = {\frac{I\left( {{hk}\quad 1} \right)}{I_{o}\left( {{hk}\quad 1} \right)}\left\{ {\frac{1}{n}{\sum\frac{I\left( {{hk}\quad 1} \right)}{I_{O}\left( {{hk}\quad 1} \right)}}} \right\}^{- 1}}$

where

-   -   I (hkl)=intensity of the (hkl) reflection    -   I_(o)(hkl)=standard intensity according to JCPDS card no 46-1212    -   n=number of reflections used in the calculation (hkl)        reflections used are: (012), (104), (110), (113), (024), (116).    -   The texture of the alumina layer is defined as follows:    -   TC(116) greater than about 1.8, preferably greater than about        2.0 and most preferably greater than about 3.0 and        simultaneously TC(012), TC(113), TC(024) all less than about        1.5, preferably less than about 1.0 and most preferably less        than about 0.5. Note that the intensities of the related (012)        and (024) reflections are also low. However, for this growth        mode, TC(104) is somewhat higher than the other background        reflections but should be less than about 1.5, preferably less        than about 1.0 and most preferably less than about 0.5 or at        least obey the following: TC(104) less than about 0.6×TC(116),        preferably less than 0.3×TC(116).

The substrate comprises a hard material such as cemented carbide,cermets, ceramics, high speed steel or a super hard material such ascubic boron nitride (CBN) or diamond, preferably cemented carbide orCBN. By CBN is herein meant a cutting tool material containing at least40 vol-% CBN. In a preferred embodiment, the substrate is a cementedcarbide with a binder phase enriched surface zone.

The coating comprises a first layer adjacent the body of CVD Ti(C,N),CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfNor combinations thereof, preferably of Ti(C,N), having a thickness offrom about 1 to about 20 μm, preferably from about 1 to about 10 μm andsaid α-Al₂O₃ layer adjacent said first layer having a thickness of fromabout 1 to about 40 μm, preferably from about 1 to about 20 μm, mostpreferably from about 1 to about 10 μm. Preferably there is anintermediate layer of TiN between the substrate and said first layerwith a thickness of less than about 3 μm, preferably from about 0.5 toabout 2 μm.

In one embodiment the α-Al₂O₃ layer is the uppermost layer.

In another embodiment, there is a layer of carbide, nitride,carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having athickness of from about 0.5 to about 3 μm, preferably from about 0.5 toabout 1.5 μm atop the α-Al₂O₃ layer. Alternatively, this layer has athickness of from about 1 to about 20 μm, preferably from about 2 toabout 8 μm.

In yet another embodiment, the coating includes a layer of κ-Al₂O₃and/or γ-Al₂O₃ preferably atop the α-Al₂O₃ with a thickness of fromabout 0.5 to about 10, preferably from about 1 to about 5 μm.

The invention is additionally illustrated in connection with thefollowing examples, which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the examples.

EXAMPLE 1

Cemented carbide cutting inserts with a composition of 5.9% Co andbalance WC (hardness about 1600 HV) were coated with a layer of MTCVDTi(C,N). The thickness of the MTCVD layer was about 2 μm. On to thislayer three different alumina layers consisting of about 10 μm α-Al₂O₃were deposited:

-   -   Layer a) contained a (116) textured layer and was deposited        according to the present invention. The detailed process data is        given in Table 1.    -   Layer b) was deposited according to the prior art.

Layer c) was deposited according to the present invention at 800° C. Thedetailed process data is given in Table 2. TABLE 1 Deposition processfor a Layer a) with (116) texture at 1000° C.: Step 1: Bonding layer Gasmixture TiCl₄ = 2.8% AlCl₃ = increasing from 0.8 to 5.2%      CH₃CN =0.5% CO = 5.8% CO₂ = 2.2% N₂ =   5% Balance: H₂ Duration 40 minTemperature 1000° C. Pressure 100 mbar Step 2: Purge Gas Ar =  100% Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 3: Pulse 1 Gasmixture TiCl₄ = 12.5%  AlCl₃ = 2.8 H₂ = Balance Duration 5 min.Temperature 1000 C. Pressure 50 mbar Step 4: Purge Gas Ar =  100% Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 5: Pulse 2 Gasmixture CO₂ = 0.05%  N₂ =  16% H₂ =  10% Balance: Ar Duration 1 minTemperature 1000° C. Pressure 100 mbar Step 6: Purge Gas Ar =  100% Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 7: Nucleationstep Gas mixture AlCl₃ = 3.2% HCl = 2.0% CO₂ = 1.9% Balance H₂ Duration60 min Temperature 1000° C. Pressure 210 mbar Step 8: Deposition Gasmixture AlCl₃ = 4.2% HCl = 1.0% CO₂ = 2.1% H₂S = 0.2% Balance: H₂Duration 520 min Temperature 1000° C. Pressure 50 mbar

Steps 3-6 were repeated 3 times. TABLE 2 Deposition process for a Layera) with (116) texture at 780° C.: Step 1: Bonding layer Gas mixtureTiCl₄ = 2.8% CH₃CN = 0.7% AlCl₃ = increasing from 0.8 to 5.4%      CO =5.8% CO₂ = 2.2% N₂ =   5% Balance: H₂ Duration 40 min Temperature 780°C. Pressure 100 mbar Step 2: Purge Gas Ar =  100%  Duration 5 minTemperature 780° C. Pressure 50 mbar Step 3: Pulse 1 Gas mixture TiCl₄ =12.5%  AlCl₃ = 2.5 H₂ = Balance Duration 5 min. Temperature 780° C.Pressure 50 mbar Step 4: Purge Gas Ar =  100%  Duration 5 minTemperature 780° C. Pressure 50 mbar Step 5: Pulse 2 Gas mixture CO₂ =0.05%  N₂ =  17% H₂ =  10% Balance: Ar Duration 2 min Temperature 780°C. Pressure 100 mbar Step 6: Purge Gas Ar =  100%  Duration 5 minTemperature 780° C. Pressure 50 mbar Step 7: Nucleation step Gas mixtureAlCl₃ = 3.2% HCl = 2.0% CO₂ = 1.9% Balance H₂ Duration 60 minTemperature 780° C. Pressure 50 mbar Step 8: Deposition Gas mixtureAlCl₃ = 4.1% HCl = 1.0% CO₂ = 2.5% H₂S = 0.9% Balance: H₂ Duration 600min Temperature 780° C. Pressure 350 mbar

Steps 3-6 were repeated three times.

EXAMPLE 2

Layers a), b) and c) were studied using X-ray diffraction. he texturecoefficients were determined are presented in Table 3. As clear fromTable 2. TC(104) is somewhat higher than the other backgroundreflections. TABLE 3 hkl Invention, Layer a Prior art, Layer bInvention, Layer c 012 0.29 0.97 0.74 104 0.59 1.10 0.97 110 0.45 0.950.20 113 0.37 0.99 0.15 024 0.41 0.96 0.51 116 3.89 1.03 3.43

EXAMPLE 3

Layers a) and b) were studied using Scanning electron microscopy. Thecross section images of the coatings are shown in FIGS. 1 and 2,respectively. The differences in microstructure and morphology areclear.

EXAMPLE 4

The layers a) and b) from the Example 1 were tested with respect to edgechipping in longitudinal turning in cast iron. Work piece: Cylindricalbar Material: SS0130 Insert type: SNUN Cutting speed: 400 m/min Feed: 0.4 mm/rev Depth of cut:  2.0 mm Remarks: dry turning

The inserts were inspected after 2 and 4 minutes of cutting. As clearfrom Table 4 the edge toughness of the prior art product wasconsiderably enhanced when the layer was produced according to thisinvention. TABLE 4 Flaking of the edge line (%) Flaking of the edge line(%) after 2 minutes After 6 minutes Layer a 0 5 (Invention) Layer b 1632

EXAMPLE 5

The layer produced according to this invention was compared with amarket leader, referred to here as Competitor X. This coating iscomposed of MTCVD Ti(C,N) and α-Al₂O₃. XRD was used to determine thetexture coefficients for these competitor coatings. Two inserts fromCompetitor X were randomly chosen for XRD. Table 4 shows the obtainedTCs for the Competitor X. The alumina layer from Competitor X exhibit arandom texture and can be compared with the present invention withstrong (116) texture, Table 3. TABLE 4 Hkl TC(hkl) 012 0.59 0.57 1040.92 0.88 110 1.71 1.90 113 0.48 0.42 024 1.12 1.12 116 1.02 1.01The X-rayed inserts from the competitor X were compared with insertsproduced according to the present invention, Layer a).

Two inserts produced according to this invention were compared with thetwo Competitor X inserts with respect to wear resistance in turning ofball bearing material Work piece: Cylindrical tubes (Ball bearings)Material: SS2258 Insert type: WNMG080416 Cutting speed: 500 m/min Feed:0.5 mm/rev Depth of cut: 1.0 mm Remarks: Dry turning Tool lifecriterion: Flank wear >0.3 mm, three edges of each variant were tested.Results: Tool life (min) Layer a 32.2 (invention) Layer a 29.5(invention) Competitor 1 14.5 (prior art) Competitor 2 15.5 (prior art)

EXAMPLE 6

Layer a), b) and c) deposited on Co-enriched substrates were tested withrespect to toughness in longitudinal turning with interrupted cuts. Workpiece: Cylindrical slotted bar Material: SS1672 Insert type:CNMG120408-M3 Cutting speed: 140 m/min Feed: 0.1, 0.125, 0.16, 0.20,0.25, 0.315, 0.4, 0.5, 0.63, 0.8 mm/rev gradually increased after 10 mmlength of cut Depth of cut: 2.5 mm Remarks: dry turning Tool lifecriteria: Gradually increased feed until edge breakage. 10 edges of eachvariant were tested.

TABLE 6 Mean feed at breakage (mm/rev) Layer a (invention) 0.40 Layer b(prior art) 0.12 Layer c (invention) 0.40

The test results show (Table 6) that layers according to the inventionexhibited clearly better toughness behaviour than the prior-art (Layerb).

EXAMPLE 7

Cubic boron nitride (CBN) insert containing about 90% of polycrystallineCBN (PCBN) were coated according to this invention and according toprior art layer discussed in Example 1. The coated CBN was compared withuncoated CBN insert in cutting of steel containing ferrite. It is knownthat B has a high affinity to ferrite and diffusion wear occurs at highcutting speeds. As shown in Table 7 the layer according to thisinvention is superior to the prior art. Work piece: Cylindrical barMaterial: SS0130 Insert type: SNUN Cutting speed: 800 m/min Feed:  0.4mm/rev Depth of cut:  2.5 mm Remarks: dry turning

TABLE 7 Life time (min) Coated CBN (Invention, layer c) 22 Coatedaccording to prior art 11 Uncoated CBN 9

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1. Cutting tool insert of a substrate at least partially coated with acoating with a total thickness of from 10 to about 40 μm of one or morerefractory layers of which at least one layer is an alumina layerwherein said alumina layer is composed of columnar α-Al₂O₃ grains withtexture coefficients a) TC(116) greater than about 1.8 b) TC(012),TC(104), TC(110), TC(113), TC(024) all less than about 1.5. The texturecoefficient TC(hkl) being defined as${{TC}\left( {{hk}\quad 1} \right)} = {\frac{I\left( {{hk}\quad 1} \right)}{I_{o}\left( {{hk}\quad 1} \right)}\left\{ {\frac{1}{n}{\sum\frac{I\left( {{hk}\quad 1} \right)}{I_{O}\left( {{hk}\quad 1} \right)}}} \right\}^{- 1}}$where I(hkl)=measured intensity of the (hkl) reflectionI_(o)(hkl)=standard intensity according to JCPDS card no 46-1212n=number of reflections used in the calculation (hkl) reflections usedare: (012), (104), (110), (113), (024), (116).
 2. A cutting tool insertof to claim 1 wherein said alumina layer is composed of columnar grainswith the length/width ratio from about 2 to about
 15. 3. A cutting toolinsert of claim 1 wherein that said substrate comprises cementedcarbide, CBN or sintered CBN alloy.
 4. A cutting tool insert of claim 1wherein the coating comprises a first layer adjacent the body of CVDTi(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVDTi(B,C.N), CVD HfN or combinations thereof having a thickness of fromabout 1 to about 20 μm and said α-Al₂O₃ layer adjacent said first layerhaving a thickness of from about 1 to about 40 μm.
 5. A cutting toolinsert of claim 1 wherein the α-Al₂O₃ layer is the uppermost layer.
 6. Acutting tool insert of claim 1 wherein a layer of carbide, nitride,carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having athickness of from about 0.5 to about 12 μm is atop the α-Al₂O₃ layer. 7.A cutting tool insert of claim 1 wherein a layer of κ-Al₂O₃ or γ-Al₂O₃is atop the α-Al₂O₃ with a thickness of from about 0.5 to about 10 μm.8. A cutting tool insert of claim 1 wherein a layer of TiN is betweenthe substrate and said first layer with a thickness of less than about 3μm.
 9. A cutting tool insert of claim 3 wherein said substrate comprisesa cemented carbide with a binder phase enriched surface zone.
 10. Acutting tool insert of claim 1 wherein the coating has a total thicknessof from about 15 to about 25 μm, the texture coefficient TC(116) isgreater than about 2.0 and the texture coefficients TC(012), TC(104,TC(110), TC(113), and TC(024) is less about 1.0.
 11. A cutting toolinsert of claim 10 wherein the texture coefficient TC(116) is greaterthan about 3.0 and the texture coefficient TC(012), TC(104, TC(110),TC(113), and TC(024) is less about 0.5.
 12. A cutting tool insert ofclaim 1 wherein the length/width ratio of the columnar grains is fromabout 5 to about
 8. 13. A cutting tool insert of claim 4 wherein saidfirst layer comprises Ti(C,N) leaving a thickness of from about 1 toabout 10 μm and said α-Al₂O₃ is from about 1 to about 20 μm.
 14. Acutting tool insert of claim 13 wherein the thickness of said α-Al₂O₃layer is from about 1 to about 20 μm.
 15. A cutting tool insert of claim6 wherein said layer of carbide, nitride, carbonitride or carboxynitridehas a thickness of from about 0.5 to about 6 μm.
 16. A cutting toolinsert of claim 7 wherein said layer of κ-Al₂O₃ or γ-Al₂O₃ has athickness of from about 0.5 to about 6 μm.
 17. A cutting tool insert ofclaim 8 wherein the TiN layer has a thickness of from about 5 to about 2μm.
 18. A method of coating a substrate with an Al₂O₃ layer wherein theα-Al₂O₃ layer is composed of columnar α-Al₂O₃ grains with a texturecoefficient TC (116) greater than about 1.8 comprising depositing a(Ti,Al) (C,O,N) bonding layer on said substrate to provide a nucleationsurface for said Al₂O₃, subjecting said nucleation surface to amodification treatment of a pulse treatment with a mixture of TiCl₄,AlCl₃ and H₂, a purge with a neutral gas and an oxidizing pulse of a gasmixture including N₂ and CO₂, repeating the modification treatment anddepositing α-Al₂O₃ having a texture coefficient TC(116) greater thanabout 1.8 at a temperature of from about 750 to about 1000° C.
 19. Themethod of claim 18 wherein the neutral gas is argon.
 20. The method ofclaim 18 wherein the mixture of TiCl₄, AlCl₃ and H₂ comprises a mixtureof about 5 to about 15% TiCl₄, from about 2 to about 4% Al₂O₃, balanceH₂.
 21. The mixture of claim 20 wherein the said pulse treatment withTiCl₄, AlCl₃ and H₂ is conducted for a time of from about 2 to about 60minutes at a temperature of from about 750 to 1000° C. and a pressure offrom about 50 to about 200 mbar.
 22. The method of claim 18 wherein theoxidizing pulse comprises a mixture of from about 0.1 to about 0.15%CO₂, from about 10 to about 40% N₂, about 10% H₂, balance Ar.
 23. Themethod of claim 22 wherein the oxidizing pulse is conducted for a timeof from about 0.5 to about 5 minutes, a temperature of from about 750 toabout 1000° C. and a pressure of from about 50 to about 500 mbar. 24.The method of claim 18 wherein the α-Al₂O₃ deposition is conducted at atemperature of from about 950 to about 1000° C.
 25. The method of claim18 wherein the α-Al₂O₃ deposition is conducted at a temperature of fromabout 750 to 950° C. at a pressure of from about 200 to about 500 mbar.26. The method of claim 25 wherein there is present a catalyzingprecursor in an amount of from about 0.5 to about 1.5%.
 27. The methodof claim 26 wherein the catalyzing precursor is H₂S or SO_(x).
 28. Themethod of claim 27 wherein the catalyzing precursor is H₂S.